The relationship between varicella (chickenpox) and scarlet fever in contemporary Hong Kong

Scarlet fever epidemics have reemerged in China, the UK, and Hong Kong. This research tests whether scarlet fever epidemics in Hong Kong are linked to varicella epidemics. Varicella infection is a known risk for invasive Group A Streptococcal infections, and historical research shows a connection between varicella and scarlet fever. This analysis examines the relationship between these two disease in Hong Kong from 2011 to 2015 and compares varicella rates before and after the reintroduction of scarlet fever. Analysis shows that scarlet fever and varicella have synchronous annual epidemic cycles, and a mathematical model of the relationship between scarlet fever and varicella is estimated. Varicella rates were unchanged by the return of scarlet fever, but annual varicella cycles may have influenced the size and timing of scarlet fever outbreaks. Vaccination policies for varicella may need to be adjusted to limit scarlet fever epidemics.